Use Notching To Smooth Armhole Facing Edges
If you've ever sewn a faced armhole edge, one of the biggest challenges is managing the abruptly curved underarm edges. Not only is this area difficult to machine stitch evenly, it also prone to lots of tension if not handled properly in the construction process. Luckily, these tension issues can be easily prevented with some tension-release notching:
After the facing is stitched to the armhole edge, the seam allowance layers should be trimmed if needed to eliminate excess bulk in the finished faced edge. In the example above, the seam allowance width is 1/2" thus it is trimmed by about 1/8". Next, clip triangle-shaped or wedge notches into the seam allowance at the armscye's most curved areas, in this case, the underarm. These notches should be clipped close to the stitching but not so close that you accidentally clip it or weaken the seam. Aside form releasing tension in the finished curved seam, these triangle cutouts will also remove excess seam allowance taht may cause unnecessary thickness and bulk in the finished faced edge.
The facing layer is then flipped to its natural position on the inside of the garment, exposing a smooth tension free faced edge. If you attempt to flip the facing to the garment's wrong side without first notching the seam allowance edges, you'll find that this is virtually impossible to do as there is too much tension and the finished edge refused to lay flat and smooth.